1. Field of the Invention
The present invention relates generally to an improved rotary rock cutter-to-journal shaft retention and thrust bearing system, and, more specifically, to such a system which not only improves the retention and thrust bearing load carrying capabilities of such a system over the prior art, but which also accurately establishes and maintains a preselected axial position of the cutter upon the supporting journal shaft. Further, this invention substantially increases the potential effective size of the radial bearing footprint, thereby improving axial stability of the cutter about its shaft.
2. Brief Description of the Prior Art
Various retention and thrust bearing systems for rotary rock cutters mounted on a journal shaft are found in the prior art.
One example of such a prior art system is found in U.S. Pat. No. 3,620,580 by Cunningham, which describes and analyzes in some detail the use of steel bearing balls arranged in a pair oppositely-disposed complementary deep ball races. One such ball race is formed in the bore of the cutter, and the other is formed in the surface of the journal shaft. The ball races are relieved so that only those retention and thrust loads which are directed radially inward, relative to the bore hole, are imposed upon the balls, and radial loads are never imposed upon the balls.
When placed under operational conditions, the balls are loaded in shear against the relatively sharp edges of the rims of the ball races. Under the extremely heavy shock loads encountered in well drilling, the sharp edges of the ball races fail by spalling away. Such spalling is not only destructive with respect to the balls and races, but also is productive of metallic debris whereby the entire bearing structure and lubrication system of the cutter-shaft combination is disadvantaged.
Further, it should be clearly understood and appreciated that within a rotary rock bit, space is at an extreme premium. A retention and thrust bearing system based upon balls occupies a relatively large volume of space while providing a very limited bearing capacity, because the loads are concentrated upon small point contact areas of both balls and races.
Another system is described in U.S. Pat. No. 2,076,003 issued to Reed, which teaches the use of a number of disks resembling thick coins which are half housed in a square groove formed within the rotary rock cutter while the remaining halves ride against a radially extended flange formed on the surface of the journal shaft.
While the retention strength of this configuration is relatively high, being equal to the shearing strength of the disks taken across the sum of their diameters, the system is disadvantaged in that it occupies a volume of space comparable to that occupied by the ball system of Cunningham. It is further disadvantaged in that the structural support of the cantilevered journal shaft is seriously weakened at the critically loaded leg-to-journal shaft junction, by the removal of a large radial segment of metal in order to permit assembly.
In U.S. Pat. No. 2,192,697 by Scott, a rotary rock cutter and journal shaft retention and thrust bearing system is disclosed which uses a primary roller radial bearing, and a second friction radial bearing with a radially extending flange formed by the journal shaft disposed in between. The secondary friction bushing is split longitudinally and fits into an annular groove formed in the surface of the journal shaft. This bushing has an annular rectangular groove formed in its outside diameter which compliments a similar groove formed in the rotary rock cutter, the pair being adapted to receive a thick snap ring. The snap ring is compressed to be loaded into the groove in the rotary rock cutter, the groove being oversize. The bushing outside diameter is tapered smaller at its distal end to facilitate entry into the snap ring at assembly.
Although the thrust bearing capacity of this design is adequate, the radial bearing capacity is low considering the inefficient use of space of both the rolers and the thick bushing.
A thrust bearing and retention design is disclosed by Boice in U.S. Pat. No. 2,661,322 which uses a complement of tapered rollers as conventionally used in roller thrust bearings.
Although the bearing capacity of this system is better than the capacity of the Cunningham system, it is substantially less than is provided by such a system as, for example, Reed. In the Boice system, thrust loads in either direction serve to put the larger end of the rollers into hard frictional contact with the outside diameter of the race formed in the rotary rock cutter, and thus against the retaining member used to block the loading hole which is formed through the outer working surface of the rock cutter. This hole through the outer surface of the rock cutter is sealed after assembly by welding. Welding is to be avoided, as a practical matter, in that such a process causes a loss of control of critical internal dimensions through warpage and distortion, and the wear resistance surface generated by heat treat processes is seriously disrupted by the welding process. Such a welded spot disrupts the integrity of the formerly single piece, metallurgically-uniform, rock cutter body and the strength of the rotary rock cutter by the introduction of undesirable stress risers in the rotary rock cutter body.
Another approach is taught in U.S. Pat. No. 2,823,083 by Welton. In this approach, symmetrical complementary 45 degree "V" grooves are formed respectively in rock cutter bore and journal shaft surface. Alternating crossed rollers which are of a length slightly less than their diameter are disposed in the opposing grooves in a configuration commonly known as a "crossed roller bearing". Such bearings are generally used to carry primary radial or linear loads along with minor thrust loads in either direction.
In this particular design, the radial capacity is helpful to compensate in part for the substantial loss of radial bearing length incurred in its use. While retention strength is relatively high in this system, being equal to the shear strength of the sum of the rollers taken diagonally through their bodies, it is disadvantaged by low thrust load capacity. Thrust loads are borne by only half of the rollers at any one time, and then only indirectly at the 45 degree angle.
Yet another system is described in U.S. Pat. No. 3,361,494 by Galle. This patent is directed to a journal shaft having a shape like an arrowhead of revolution, with a truncated point and relatively small barbs. This arrowhead-like shape is reproduced in the rotary rock cutter bore. A large secondary cylindrical cut is taken on the upper unloaded side of the journal shaft tpering from mere contact at the proximal end of the shaft to a depth of about two "barbed heights" at the barb location, and having substantially the same radius as the basic journal shaft. This permits assembly of the cutter over the remaining lower barb by tipping the vertex of the conical cutter downward during the start of assembly, then upward to alignment with the journal shaft when it is fully in place thereon. A retention pin having an end resembling the missing barb is then positioned through a small angled bore formed through the journal shaft to retain the cutter in place by partially replacing the missing barb.
In this arrangement, Thrust loads are supported by a narrow area of contact with the barb on the lower loaded side of the journal shaft, a point contact with the retaining pin at the upper side of the journal shaft, and virtually no contact at all on the sides of the journal shaft. It is clearly a disadvantage of this system that something over fifty (50) percent of the journal shaft is machined in relief and thus can not bear any load at all.
Welton, in U.S. Pat. No. 3,746,405, describes a retention system based on a stout split snap ring. Proportioned as pictured, the radial bearing is enlarged in diameter, but not in length. As a result, the thrust load bearing capacity is relatively low and the ultimate retention strength is relatively high. Because of this limitation, this design is thought not to have been used commercially
Vezirian discloses a system in U.S. Pat. No. 4,145,094 based upon a thick bushing which provides both radial and thrust bearing surfaces, grease reservoir, and a retention device. This bushing is split longitudinally into three sections, one of which is reduced in outside radius. The inside radii of all three sections closely fit the uniform diameter of the journal pin. The thin section provides a grease reservoir and facilitates the assembly of the three sections into an annular recess in the rotary rock cutter bore where they provide twin thrust bearing surfaces, and the radial bearing surface is provided by the two thicker sections. At assembly, the straight journal pin is passed through closely fitting bores in the split bushing and in the supporting leg. With the thin section of the bushing positioned on the unloaded upper side of the journal shaft, an electron welding beam is passed circumferentially around the journal pin thus welding the pin to the three sections of the split bushing and to the supporting leg.
The journal pin is substantially less in diameter than is the journal bearing, thus providing a weak cantilever support for the cutter assembly. Even with the fast efficient electron beam welding, the surrounding grease seal may be thermally damaged by the process.
U.S. Pat. No. 4,176,724 by Vezirian teaches the assembly of the necessary structures by means of an electron welding beam passing through a small hole in either the journal shaft or the rotary rock cutter. A segmented retaining ring having a rectangular cross section is welded to the member not bearing the access hole by rotating the cutter relative to the journal shaft while the welding is being accomplished. Finally the access hole is closed by a threaded plug or by welding.
In a later patent issued to the same inventor a further refinement in the electron beam assembly process is disclosed. In this system a headed journal bearing bushing is captured in the rotary cutter bore by welding in a mating bearing bushing providing radial and thrust bearing surfaces therebetween. Then a straight bearing pin passing through both the supporting leg and the inner bushing is welded to both members by the electron beam. This design shares the weakness of the cantilever mentioned in the previous patent.
Another rock cutter retention system containing a wire split snap ring of very light circular cross-section is used in U.S. Pat. No. 4,344,658 by Ledgerwood. This wire ring is loaded into a journal shaft groove having a distal side flaring conically outward. At assembly the wire ring expands into a semi-circular groove in the rotary rock cutter to a depth just over one half of the wire diameter. In-thrust loads reacting against the conical outer wall of the groove in the shaft serve to seat the wire ring ever more tightly in the groove in the rotary cutter. Both thrust capacity and retention strength in this system are very low. Furthermore, just as in the ball retention system of Cunningham, the design is disadvantaged by the impression of these loads upon sharp edges of the retaining grooves which leads to the rapid formation of undesirable detritus within the lubricated bearing system and results in a weak axial location of the rotary cutter and contributes to early cone wobble.
Various other patents exist for retention systems which are of lesser interest due to such various problems as the uneconomical use of cutter internal space, low thrust capacity, poor retention support, or excess complexity for use in the difficult drilling environment. Examples of such prior art are represented by: U.S. Pat. No. 4,181,377 by Oelke, U.S. Pat. No. 2,697,014 by Boice, U.S. Pat. No. 3,193,028, by Radzimovsky, U.S. Pat. No. 4,136,748 by Dickerhoff, U.S.Pat. No. 4,157,122 by Morris, and U.S. Pat. No. 4,444,518 by Schramm, et al.